The present invention relates to a device for introducing small amounts of mercury into fluorescent lamps and to the lamps thus obtained.
As it is known, the fluorescent lamps require small amounts of mercury for their working. As a result of the technological development, and of international standards more and more strict about the industrial use of potentially harmful substances such as indeed mercury, the maximum amount of this element being used in the lamps has been reduced in the last years from 20-30 mg per lamp to about 3 mg per lamp, and at present some manufacturers demand to be able to dose even smaller amounts of mercury.
Many of the conventional methods for dosing the mercury are not capable to meet these demands.
For example, the mercury volumetric dosing in the lamps in form of liquid droplets of the pure element is by now practically inapplicable: in fact, a mercury droplet of 1 mg has a volume of about 0.07 xcexcl, and the volumetric dosing of so small element amounts is exceedingly complex, and anyhow the reproducibility of the element weight for the following dosings is very low. Furthermore, the dosing of liquid mercury directly into the lamps causes pollution problems of the working environment due to the high vapor pressure of this element.
Other methods involve the introduction of mercury into the lamps in form of pure element contained in little glass capsules, as disclosed e.g. in patents U.S. Pat. No. 3,794,402, U.S. Pat. No. 4,182,971, and U.S. Pat. No. 4,278,908, or in little capsules made of metal, as disclosed e.g. in patents U.S. Pat. No. 3,764,842, U.S. Pat. No. 4,056,750, U.S. Pat. No. 4,282,455, U.S. Pat. No. 4,542,319, U.S. Pat. No. 4,754,193 and U.S. Pat. No. 4,823,047. However, by using these little capsules, the aforementioned problem of an accurate and reproducible dosing of very small amounts of liquid mercury is not solved.
U.S. Pat. No. 4,808,136 and patent application EP 568,317 disclose the use of pellets or little spheres, made of porous material, being impregnated with mercury, which is subsequently released by heating once the lamp is sealed. However, also these methods need complex operations in order to load the mercury into the pellets, and the released mercury amount is hardly reproducible. Furthermore, by these methods the problem of mercury vapors polluting the working environment is not solved.
U.S. Pat. No. 3,657,589, in the applicant""s name, represents the closest prior art and discloses the use of intermetallic mercury compounds having the general formula TixZryHgz, wherein x and y range from 0 to 13, the sum (x+y) ranges from 3 to 13 and z is 1 or 2; these compounds will be hereinafter also referred to as mercury releasing compounds. The dosing of small mercury amounts by means of any of these compounds is rather simple, since it is possible e.g. to laminate powders of the compound on a metal tape, and, by adjusting thickness and width of the powder track on the tape, predetermined values may be obtained for the linear loading, measured as mg of mercury per tape centimeter. The use of the compound Ti3Hg, manufactured and sold by the applicant under the tradename St505, is specially advantageous; in particular, the compound St505 is sold in form of powder compressed in a ring-shaped container, or as powder compressed in pellets or tablets, under the trademark STAHGSORB(copyright), or in form of powders laminated onto a metal tape, under the trademark GEMEDIS(copyright). Once the compound is introduced into the lamp, e.g. in form of a piece of laminated tape, the mercury is released upon heating the compound at a temperature higher than 550xc2x0 C., by a so-called xe2x80x9cactivationxe2x80x9d operation; the heating treatment may be carried out e.g. by irradiating with radiofrequencies from outside the lamp the tape carrying the compound. However, the problem found by using these compounds is that the mercury released during the activation step is about 30-40% of the total mercury. This results in the necessity of introducing into the lamp an amount of mercury (in form of any of the aforementioned releasing compounds about 2-3 times greater than the amount required for the lamp working. The mercury in excess remains in the lamp as its service life ends, possibly resulting in disposal problems.
Published patent application EP 91,297 discloses a device for the mercury release which is formed of a metallic container completely closed, wherein there is a mixture composed of Ti3Hg or Zr3Hg and powders of nickel (Ni) or copper (Cu). According to this document, the addition of Ni and Cu to the mercury releasing compounds causes the system melting, thus favoring the release of nearly all the mercury in a few seconds. The container is closed by means of a steel, copper or nickel sheet, which is broken during the activation by the mercury vapor pressure generated in the container. This solution is not completely satisfying, because the mercury discharge is violent, possibly resulting in damages of tube portions, and furthermore the container assembling is very complex, requiring welding on small-size metal members.
U.S. Pat. No. 5,520,560 and published patent applications EP 691,670 and EP 737,995, all in the applicant""s name, disclose combinations of materials comprising any of the aforementioned TixZryHgz, compounds and an alloy of copper with one or more elements selected among tin, indium, silver, silicon or rare earths. These copper alloys act as promoters for the mercury emission, allowing an element release greater than 80% during the activation step. These combinations of materials solve the problems affecting other methods for introducing mercury into the lamps, and allow the dosing of small mercury amounts, with the sole drawback of requiring a second component besides the mercury releasing compound.
It is the object of the present invention to provide a device for accurately and reproducibly introducing small mercury amounts into fluorescent lamps, without having to use a second component, as well as to provide the lamps obtained by use of the device.
According to the present invention, these objects are achieved by using a mercury releasing device which is formed of a metallic container being capable of retaining powders but not completely closed, containing at least a mercury releasing compound selected among the TixZryHgz, compounds, wherein x and y range from 0 to 13, the sum (x+y) ranges from 3 to 13 and z is 1 to 2.
The container of the device of the invention may have any shape, provided it is capable of retaining the powder particles of the TixZry,Hgz, compound used, and provided the container is not completely closed, having on at least a portion of its surface micro-holes or slits for the mercury discharge.
As already said, the TixZryHgz compounds, when used in the known devices, in form of powder pellets, contained in open containers or laminated onto tapes, during the activation step release mercury amounts not greater than 40% of the element content. It has been found that, when these compounds are used alone in the devices of the invention, the mercury yield during the activation step is at least 80% of the total amount. It is therefore possible to introduce in the lamp a smaller mercury amount with respect to the known devices comprising the TixZryHgz compounds, being practically the mercury amount actually required.